Cannot set dimensions
As @Markus von Broady pointed out, editing an objects dimensions is available in the N-Panel (hit n key) but not in edit mode, you need to be in object mode.

Changing any of the dimensions will also change the scale, so be sure to apply scale if you're doing any deformations, physics, etc.
There are other ways to get accurate dimensions while modeling in edit mode but that's for another question :)